1. Field of the Invention
The present invention relates to a method and apparatus for fabricating and using volume holographic wavelength blockers of high optical density and narrow bandwidth. Wavelength blockers are used to attenuate the signal of a pump source such as lasers while letting a scattering signal such as but not limited to fluorescence or Raman to go through. Thick reflective volume holographic elements (>typ. 0.1 mm thickness) have narrow rejection band but have limited attenuation of the order of optical density of 1 to 2. It is desirable to have a narrow spectral band rejection in conjunction with high attenuation reaching at least an optical density 6 for Raman spectroscopy for example.
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2. Background Art
Wavelength blockers, also called notch rejection filters, are an essential component in Raman and fluorescence instruments. The purpose of the wavelength blocker is to greatly attenuate the backscattered light from the laser illuminating a sample under test, while letting the faint Raman spectrally shifted signature pass through. Two non-dispersive filter technologies are currently used for the wavelength blocker: holographic and thin film. Commercial holographic notch filter technology uses holographic recording in a thin film of dichromated gelatin to produce a notch filter with 3 dB bandwidth of 350 cm−1 and optical density of 6. Commercial thin film technology uses deposition of many layers to obtain a 3 dB bandwidth of approximately 600 cm−1 and optical density of 6. Both technologies provide a compact size wavelength blocker element with a 10 mm aperture diameter and several millimeters thickness. However both notch filter technologies are limited to observing Raman spectral shift above approximately 350 cm−1.
The Raman signal in the low frequency shift region, i.e near the frequency of the excitation laser, contains critical information about the molecular structure. For example carbon nanotubes exhibit vibration modes in the range of 150 cm−1 to 200 cm−1 depending on their size. Relaxation in liquids, solutions and biological samples exhibit Raman shift in the range between 0 and 400 cm−1. U.S. Pat. Nos. 5,684,611 and 5,691,989 describe the use of reflective volume holographic filters (VHG) with millimeters thickness as filters producing 3 dB bandwidth of the order of 10 cm−1. VHGs produced in a glass material are now commercially available and show long lifetime, high efficiency and excellent transmission in the red and near infrared. The photosensitive glass can contain for example silicon oxide, aluminum oxide and zinc oxide, fluorine, silver, chlorine, bromine and iodine, cerium oxide. Composition and processes for manufacturing the photosensitive glass are described in U.S. Pat. No. 4,057,408, the disclosure of which is incorporated herein by reference.
Large area (30×30 mm) reflective VHGs are restricted to the millimeter range thickness due to the material absorption. The optical density (O.D) achievable is therefore limited to O.D near unity (i.e ˜90% efficiency) with thickness of 1.5 mm and transmission of 97 to 98% away from the notch in the near infrared.
By carefully individually aligning a cascade of VHGs, researchers have shown that the optical density can be added up: a cascade of 4 VHGs with each exhibiting an optical density of one yields a compounded notch with an optical density of 4. Commercial instruments comprising individual alignment fixtures for each VHG exhibit an optical density ranging from 4 to 6 with bandwidth of 10 cm−1. However, there are several drawbacks to this approach:                1. The alignment procedure is complicated and required for each VHG separately.        2. The footprint is large (˜100 cm3) and as such not suitable to replace standard notch filters in existing Raman instruments.        3. The surface of each VHG contributes to Fresnel reflection loss.        4. Upon rotation of the assembly, the individual VHGs spectrally shift at different rates, thus reducing severely the optical density and broadening the overall blocker bandwidth.        
The technology utilized to observe the Raman signal close to the laser excitation (>9 cm−1) is based on cascading dispersive spectrometers. The cascaded spectrometers are bulky (˜1 m2), expensive (˜$100K) and of moderate transmission (˜50%).